Procedures for the alignment of vented loudspeakers, utilizing standard formulae for cabinet tuning, have been thoroughly expounded by A. N. Thiele and Richard H. Small. However, interest in the dynamic conditions inside the enclosure remains high, and has led to innovative vent designs and the development of techniques for modifying the environment within loudspeaker cabinets to alter their effective sizes.
The Venturi Vent design comes readily to mind, but perhaps a better know example is the Isobaric System in which one driver, located deep inside the enclosure, creates the acoustic environment for a second, external driver that radiates the sound. Needless to add, this latter example seems wasteful of driving units.
Since the pressure distribution inside a loudspeaker enclosure becomes increasingly nonuniform above 50 Hz (Small, 1971), in the dynamic state in which a wide band of audio frequencies is being reproduced, there already exists, within the enclosure, conditions that allow for optimizing the low frequency performance of the system through careful design and placement of the vent or vents. In this regard the relationship between vent terminations and the pressure distribution inside loudspeaker enclosures remains to be fully explored. For example, vent termination away from high pressure areas and towards areas of relatively more rarefied air should have the effect of tuning a relatively larger box, and vice versa. In such cases, standard formulae for tuning, based on the principles of the Helmholtz resonator, are likely to yield results that require modification by a correction factor to optimize performance.
Furthermore, although the use of absorbent materials to change conditions in closed-box systems has been thoroughly discussed (Moir, 1962) and (Small, 1971), the application of such materials to modify the behaviour of the air mass in a vented system, and thereby vary the tuning, has received far less attention. This latter concept, however, is fully embraced in the present invention.
It is well known that the pressure distribution inside a loudspeaker enclosure is uniform below approximately 50 Hz but becomes increasingly non-uniform above that frequency. Intuitively, one senses that this could influence the techniques employed in the tuning of vented systems. Yet, surprisingly little has been said concerning the possibility of exploiting this phenomenon of pressure distribution by using vent size and placement to optimize performance at low frequencies. An important design consideration would be to avoid the restrictiveness of high pressure areas in the placement of vents; or to state the converse, the benefits of rarefication for simulating a larger enclosure should be investigated.
To pursue this line of reasoning further, it is to be noted that although pressure inside an enclosure is uniform below 50 Hz, or at frequencies where most vented enclosures would normally be tuned, in the strictest of senses this state can only exist in a bandwidth sweep. In reality, many different frequencies are present at once in the reproduction of musical sounds, and forces tending towards pressure uniformity and non-uniformity occur simultaneously. Small observed that pressures tend to be higher than average near the back panel(s) and lower than average near the driver(s). He implied that pressure changes near the geometrical center of the enclosure are less extreme.
Other background an which can be regarded as useful includes U.S. Pat. document No. US-A-4 837 839. This patent describes a compact speaker assembly with an improved low frequency response. This patent merely discloses a speaker as opposed to a speaker system which is the subject of the present invention. In this patent, a speaker transducer assembly is described which is comprised of a pair of speaker diaphragms superimposed on each other and separated by a intermediate partition or baffle. The spaces between the speaker diaphragms and the partition are vented to the outside of the speaker frame by suitable vent openings. This transducer is designed for use generally in motor vehicles or for use in a non-enclosure type mounting.